Electric motors usually have coils with windings made of wire. The windings can in particular be stator windings, but can also be rotor, or armature, windings.
Basically, only one wire is used for the windings of such coils. However, in particular, in the field of refrigerant compressors, two identical wires, each of smaller diameter, for example, 0.5 mm, which are connected electrically in parallel, are used instead of one somewhat thicker wire, with a diameter of 0.7 mm, for example. This may prove necessary, for example in the case of a stator whose mounting slots for accommodating the windings have a slot opening that is not large enough to insert the somewhat thicker wire.
The coils or windings typically consist of wires made of copper (Cu) or aluminum (Al). Cu has the advantage of a lower electrical resistivity ρ than Al, so that compact electric motors with high power and high efficiency can be realized. On the other hand, Al has the advantage of lower material costs. A limiting factor for using Al in particular is the space available for the coil or windings, since an Al wire has, for a given length and given electrical resistance R, a cross-sectional area that is about 65% greater than that of a Cu wire.
If therefore, for example, in the case of a stator winding with Cu wire, the mounting slots each have a mounting cross section with a mounting cross-sectional area AAQ that is filled to 61% or more by the winding—in this connection one also speaks of the degree of filling, said coil can no longer be replaced by a coil with Al wire. This means that, to achieve a desired power and/or a desired efficiency of the electric motor, although the mounting slots of the stator are nowhere near completely filled with the Cu wire, it will not be possible to replace the Cu wire with an Al wire, so that comparably high material costs must be accepted.